The buildings sector of the United States accounts for approximately 40% of the U.S. primary energy consumption and 39% of the U.S. carbon dioxide emissions. To cope with this challenging situation, efforts are needed to improve energy efficiency of U.S. buildings, which will not only save money for both homeowners and business owners, but also reduce the environmental impacts of energy use.
One approach to address these issues has been to incorporate phase change materials (PCMs) into construction materials to enhance the building's energy efficiency through Thermal Energy Storage (TES) and thermal regulation. PCMs change their phase from solid to liquid and vice versa at their phase change temperatures with large amount of energy absorbed or released. Thermal inertia (mass) of the building can be significantly increased by integrating PCMs into construction materials. PCMs have been considered as a promising method of TES in terms of narrowing the gap between the peak and off-peak loads of energy/electricity demand, reducing diurnal temperature fluctuations, and utilizing the free cooling at night for day peak cooling load shaving.
Two primary methods have been used to incorporate PCMs into construction materials: (1) microencapsulation of PCMs and (2) form-stable PCMs composites. In the first method, PCMs are encapsulated within a protective polymer shell. The produced microencapsulated PCMs can preserve PCMs as long as possible through the heating/cooling cycles. This microencapsulation method increases the heat transfer area, decreases the reactivity of the PCMs, limits the interaction with the construction materials, enhances the low heat conductivity, and facilitates the handling of the PCMs. However, it also suffers a few drawbacks preventing practical applications of the PCMs in construction materials. For example, the protection shell is made of polymers that usually have low mechanical stiffness and strength. As a result, the mechanical stiffness and strength of the construction materials can be reduced significantly by adding the microcapsules. The microcapsules can also been easily broken during the mixing of concrete, leading to leaking of the PCMs. The polymeric shell also has low chemical and thermal stability. It can be deteriorated by UV light, oxidation, and other aggressive chemicals. It can also lose its stability when temperature exceeds its glass transition temperature. The polymer shells can also be flammable, and therefore cannot be adopted by the building industry. Further, the thermal conductivity of the polymer shells is often very low, making thermal exchange between the PCMs inside the shell and the outside environment much more difficult.
In the second method, PCMs are first absorbed into porous materials such as light weight aggregates and diatomite particles to form stable composites, which are then added into the construction materials. When using porous particles to absorb PCM there are no protective layers on the surface of the composites. As a result, PCMs can still leak from the porous material once the temperature exceeds the phase change materials, leading to reduction or loss of the claimed thermal storage capacity.
Similar approaches have been tried when introducing materials other than PCMs into construction materials. This is especially prevalent when introducing admixtures into concrete. Incompatibility between the admixtures and hydration of cement is a major problem in the manufacture of concrete when the admixture is directly added into the mix. For example, water reducers, the most commonly used admixtures in concrete can have undesirable side effects such as rapid loss of workability, excessive quickening/retardation of setting, reduced rates of strength gain, and changes in long term behavior. Similarly, shrinkage reducing admixtures, which are used to reduce drying and autogenous shrinkage in concrete elements, can also cause side effects in concrete as they reduce the rate of cement hydration and strength development in concrete.
As a major ingredient of concrete, water is also used as an admixture in high strength concrete (HSC) to reduce autogenous shrinkage of the concrete through internal curing. Autogenous shrinkage is mainly caused by the capillary tension in the pore fluid caused by self-shrinkage. In the case of HSC with a water to cement ratio (W/C) below 0.3, the autogenous shrinkage can account for more than 50% of the total contraction deformation. Serious cracking can be induced in early-age concrete by autogenous shrinkage. These cracking problems cannot be mitigated through conventional full water curing because of HSC's compact pore structure and very low permeability. To minimize or eliminate autogenous shrinkage, additional moisture has to be provided within the concrete when it is needed. This additional moisture is essentially used as an admixture in concrete. However, it cannot be added directly into concrete during mixing because the compressive strength of HSC can be significantly reduced.
Undesirable interaction with cement hydration can prevent applications of some other admixtures in concrete. For example, bioactive agents have been shown to prevent corrosion of stainless steel and aluminum. They provide an eco-friendly method to prevent the corrosion in concrete. However, when these bioactive agents are simply mixed in with concrete, the 28-day compressive strength of the concrete was reduced by more than 60%. This is because the bioactive agents can cover the surface of cement particles and therefore prevent the cement particle from reacting with water, resulting in less CSH produced and much lower compressive strength.
As with PCMs, polymer based microcapsules or porous composites have been tried as a way incorporate admixtures into concrete without imparting undesirable effects caused by interactions with the admixture and concrete. For example, compositions that modify viscosity, impart antimicrobial properties, improve corrosion or fire resistance, or modify the water needed have been microencapsulated or adsorbed into porous composites and then mixed with concrete. As noted, however, these methods can have drawbacks such as breakage of the microcapsule, high manufacturing cost, leakage of the admixture, poor delivery of the admixture, or simply poor performance.
What are thus needed are new compositions and methods that can be used to incorporate PCMs and other admixtures into building materials such as concrete. The compositions and methods disclosed herein seek to address these and other needs.